fix(harness): formatValue's truncation marker could push output past VALUE_CAP

.changeset/fix-formatvalue-budget-overrun.md

@@ -0,0 +1,5 @@
+---
+"@sapiom/harness": patch
+---
+
+Fix a debug-context helper that could return output slightly longer than its configured character budget.

packages/harness/web/src/lib/extract-step-context.test.ts

@@ -154,14 +154,18 @@ describe("extractStepContext — input/output", () => {
     expect(ctx).toContain("[object Object]");
   });
 
-  it("truncates a very large value and marks the truncation", () => {
+  it("truncates a very large value and marks the truncation, bounding the total output", () => {
     const step: StepView = { id: "s1", name: "big", status: "passed" };
     const huge = "z".repeat(5000);
     const ctx = extractStepContext(step, { output: huge });
     expect(ctx).toContain("… (truncated, 5000 chars total)");
     // The head is kept; the value is not pasted whole.
     expect(ctx).not.toContain(huge);
-    expect(ctx).toContain("z".repeat(2000));
+    // The marker's own length has to come out of VALUE_CAP too, so slightly
+    // less than 2000 chars of content survive -- the total (content +
+    // marker) is what's bounded, not the content alone.
+    expect(ctx).toContain("z".repeat(1968));
+    expect(ctx).not.toContain("z".repeat(1969));
   });
 });
 

packages/harness/web/src/lib/extract-step-context.ts

@@ -103,7 +103,9 @@ export function formatLatency(ms: number): string {
  * strings pass through verbatim; anything JSON can't represent (a cycle, a
  * bigint) falls back to `String(value)` so the builder never throws. When the
  * result exceeds the cap it is head-kept (the shape and leading content matter
- * most for a value) with an explicit truncation marker.
+ * most for a value) with an explicit truncation marker, and the TOTAL output
+ * (content + marker) is what respects the cap — not the content alone, or the
+ * marker itself would push the result past VALUE_CAP.
  */
 function formatValue(value: unknown): string {
   let text: string;
@@ -117,10 +119,14 @@ function formatValue(value: unknown): string {
       text = String(value);
     }
   }
-  if (text.length > VALUE_CAP) {
-    return `${text.slice(0, VALUE_CAP)}\n… (truncated, ${text.length} chars total)`;
-  }
-  return text;
+  if (text.length <= VALUE_CAP) return text;
+  // Unlike a "chars dropped" marker, this one reports the original total
+  // length, which doesn't change with where we slice -- so the marker's
+  // length is fixed and a single subtraction (no converging loop needed)
+  // correctly reserves room for it within VALUE_CAP.
+  const marker = `\n… (truncated, ${text.length} chars total)`;
+  const sliceLen = Math.max(0, VALUE_CAP - marker.length);
+  return `${text.slice(0, sliceLen)}${marker}`.slice(0, VALUE_CAP);
 }
 
 /**